A problem inherent in the use of corrosion resistant fasteners, such as zinc plated nails, is that where such nails are driven into a substrate or workpiece, there is serious risk of damaging the zinc plated surface, thus risking an accelerated breakdown of the corrosion resistant protection afforded by the various corrosion resistant treatments. This risk of damage is present whether the nails are driven by hand hammering or by power tool insertion. In the case of power tool driven nails there is an additional risk of damage to the corrosion resistant surface in the course of the manufacturing operation of assembling groups of nails in a form suitable for use in power tool applications. This necessary assembly operation of the nails is commonly known as collating. Corrosion resistant nails damaged in the collating manufacturing operation will exhibit characteristics of premature failure by corrosion of the exposed fastener substrate, commonly steel.
One particularly useful method of imparting corrosion resistance to ferrous substrates, such as nails, is by applying a metal flake, such as zinc flake, using a hexavalent chromium salt in an aqueous dispersion. The metal flake is usually applied by immersion and is heat converted by baking to an adherent metallic gray finish. This process is disclosed in U.S. Pat. Nos. 3,671,331; 3,687,738; 3,907,608 and 4,020,220. This process utilizing a metal flake, such as zinc flake, is referred to as "Dacrotizing" and such "Dacrotized" nails have been shown by ASTM test procedures to have corrosion resistance performance equal to or greater than conventional corrosion resistance treatments for nails, such as zinc electroplating, mechanical zinc plating, or hot dip zinc galvanizing with or without subsequent chromate rinse treatment. Since the "Dacrotized" surface is harder than the zinc surface of the conventional methods of corrosion resistant treatment, it is more susceptible to abrasion damage during manufacturing or driving and it would be a desirable improvement to prevent such damage or failure from occurring.
The invention disclosed herein solves the above mentioned problems of protecting the corrosion resistant surface treatment, such as zinc coating, from damage in the collating manufacturing operation and from damage due to impact on driving the nails, or other fasteners, into a substrate. A further improvement as a result of this invention is that the long term corrosion resistance of the treated nails is significantly enhanced well beyond what could reasonably or normally be expected without the beneficial protective treatment afforded by the polymeric coating of the present invention. Experimental comparative test data demonstrate this significant improvement.
Although "Dacrotized" corrosion resistant treated nails are used as the main example of the improvement afforded by this invention, the invention is not necessarily limited to a "Dacrotized" substrate but may include other types of corrosion resistance treated substrates.